This invention relates to the control of linefeed distances in a heated, belt-type system for advancing print media through a printer.
An ink-jet printer includes at least one print cartridge that contains ink within a reservoir. The reservoir is connected to a print head that is mounted to the body of the cartridge. The print head is controlled for ejecting minute droplets of ink from the print head to a sheet of print medium, such as paper, that is advanced through the printer.
Many ink-jet printers include a carriage for holding the print cartridge. The carriage is scanned across the width of the paper, and the ejection of the droplets onto the paper is controlled to form a swath of an image with each scan. Between carriage scans, the paper is advanced so that the next swath of the image may be printed. Sometimes, more than one swath is printed before the paper is advanced. In some printers, a stationary print head or array of print heads may be provided to extend across the entire width of the paper that moves through the printer.
The relative position of the print head(s) and paper must be precisely maintained to effect high-resolution, high-quality printing. This precision is especially important in the region known as the xe2x80x9cprint zonexe2x80x9d of the printer, which is the space where the ink travels from the print head to the print media. Thus, in the course of advancing the print media between swaths, encoders and associated servo systems are employed for controlling the precise advance of the media. This incremental advance is commonly called xe2x80x9clinefeed. xe2x80x9d Thus, the control of the amount of the advance, the linefeed distance, is critical for high print quality.
One method of securing print media, such as a sheet of paper, for movement through a printer is to direct the paper against one side of a perforated belt. Vacuum pressure is applied to the other side of the belt and, thus, through the belt perforations to secure the paper to the belt. The belt, with secured paper, is moved relative to the print head and through the print zone where ink is printed on the paper.
The belt may be configured as an endless loop and secured between a pair of rollers that are mounted to the printer to drive the belt under tension. The upper surface of the belt transports the paper toward the print zone. The porous belt moves over a support surface that includes vacuum ports through which the vacuum pressure is applied to the belt and to the paper that is carried by the belt.
Any of a variety of encoder mechanisms can be employed for controlling linefeed distances. Typically, a rotary encoder is connected to the belt drive roller. The information provided by the encoder, in combination with information produced by a media edge sensor, is processed by the printer controller to control the linefeed distance between swaths. Such processing may account for runout errors or the like, but otherwise assumes that there is no movement of the media relative to the belt that carries it.
Another important factor affecting the print quality of ink-jet or other liquid-ink printers is drying time. The print media movement must be controlled to ensure that the liquid ink dries properly once printed. If, for example, sheets of printed media are allowed to contact one another before ink is adequately dried, smearing can occur as a result of that contact.
Heat may be applied to the print media in order to speed the drying time of the ink. An effective way to heat the print media is by conduction, in a manner that will not overheat the print head nor interfere with the trajectory of the droplets expelled from the print head. This can be accomplished by heating the underside of the belt by conduction, which heat is thus transferred to the media carried by the belt.
Normally, print media carries at least some moisture with it. For example, a sealed ream of standard office paper comprises about four and one-half percent moisture. High amounts of moisture in the media, such as paper, may be present in humid environments. Thus, the paper shrinks as the applied heat dries the paper.
The moisture in the paper may be substantially removed by the heated belt before the leading portion of the media passes into the print zone. Nonetheless, the shrinkage or contraction that occurs in the portion of the media that is outside of the print zone (xe2x80x9cupstreamxe2x80x9d of the print zone) can tug on the dry portion of the media by an amount sufficient to cause the media to shift slightly in the region in or near the print zone. This shifting would occur, for example, where the overall vacuum force applied to the media upstream of the print zone (that is, where the media shrinks as it dries) is greater than the vacuum force applied to the remaining portion of the media. The term xe2x80x9cupstreamxe2x80x9d means in the direction opposite to the direction of paper movement.
This shifting movement of the media in the vicinity of the print zone introduces errors in the calculation of the linefeed distances. The encoder, which follows belt or drive roller motion, does not track the movement of the shrinking paper relative to the belt. Such errors lead to degradation in print quality as a result of artifacts known as xe2x80x9cbanding.xe2x80x9d
The present invention is generally directed to techniques for preventing the forces attributable to shrinkage of print media from shifting the media relative to the belt in the vicinity of the print zone.
In one approach to the invention, a xe2x80x9chold zonexe2x80x9d is identified immediately upstream of the print zone. In the hold zone a mechanism is employed for securing the media to be immovable relative to the belt. The hold mechanism provides sufficient force to overcome tension transmitted to the dry portion of the media as a result of the shrinkage in the remaining, upstream portion of the media.
In one embodiment, the hold mechanism is a roller that pinches the media against the belt.
In another embodiment, the amount of vacuum pressure applied to the belt is regulated so that the suction force applied to the media in the hold zone is greater than the suction force applied to the media in the portion upstream of the hold zone.